Method and apparatus for storing liquefied natural gas

ABSTRACT

Systems and methods for storing liquid natural gas is provided. In one embodiment, the natural gas vapor in the storage tank containing the liquefied natural gas may be directed a heat exchange unit attached to the storage tank, cooled by a refrigerant in the heat exchange unit, and then returned to the storage tank.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Patent Application Ser. No. 61/365,129, filed Jul. 16, 2010. This application is also a continuation-in-part of U.S. patent application Ser. No. 12/765,750, filed on Apr. 22, 2010. Each application is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention generally relate to systems and methods for storing liquefied natural gas. More particularly still, embodiments of the present invention relate systems and methods for minimizing losses due to vaporization of the liquefied natural gas during storage.

2. Description of the Related Art

Natural gas is a known alternative to combustion fuels such as gasoline and diesel. One benefit of natural gas as a fuel over gasoline or diesel is that it is a cleaner burning fuel. Additionally, natural gas is considered to be safer than gasoline or diesel because natural gas will rise in the air and dissipate, rather than settling. However, the production of natural gas has various drawbacks such as higher production costs and the subsequent emissions created by the use thereof. Therefore, much effort has gone into the development of natural gas as an alternative combustion fuel.

In addition, due to its clean burning qualities and convenience, natural gas has become widely used in a variety of applications, such as heating homes. Many sources of natural gas are located in remote areas, great distances from any commercial markets for the gas. Normally a pipeline is available for transporting the natural gas to commercial markets. When pipeline transportation of natural gas is not feasible, however, it is desirable to convert the natural gas into LNG for transport and storage purposes. The primary reason for this is that the liquefaction enables the volume of natural gas to be reduced by a factor of about 600. While the capital and running costs of the systems required to liquefy the natural gas are very high, they are still much less than the costs of transporting and storing unliquefied natural gas. In addition, it is much less hazardous to transport and store LNG than unliquefied natural gas.

Conventionally, two of the known basic cycles for the liquefaction of natural gases are referred to as the “cascade cycle” and the “expansion cycle.” The cascade cycle typically consists of a series of heat exchanges with the feed gas, with each exchange being at successively lower temperatures until the desired liquefaction is accomplished. The levels of refrigeration are obtained with different refrigerants or with the same refrigerant at different evaporating pressures. Although the cascade cycle may have a relatively low operating cost, the cascade cycle generally requires relatively high investment costs for the purchase of heat exchange and compression equipment. Additionally, a liquefaction system using a cascade cycle requires a large footprint for its equipments.

In an expansion cycle, gas is conventionally compressed to a selected pressure, cooled, and then allowed to expand through an expansion turbine, thereby producing work as well as reducing the temperature of the feed gas. The low temperature feed gas is then heat exchanged to effect liquefaction of the feed gas. Conventionally, such a cycle has been seen as being impracticable in the liquefaction of natural gas since there is no provision for handling some of the components present in natural gas which freeze at the temperatures encountered in the heat exchangers, for example, water and carbon dioxide.

Additionally, to make the operation of conventional systems cost effective, such systems are conventionally built on a large scale to handle large volumes of natural gas. As a result, fewer facilities are built making it more difficult to provide the raw gas to the liquefaction plant or facility as well as making distribution of the liquefied product an issue. An additional problem with large facilities is the cost associated with storing large amounts of fuel in anticipation of future use and/or transportation. Not only is there a cost associated with building large storage facilities, but there is also an efficiency issue related therewith as stored LNG will tend to warm and vaporize over time creating a loss of the LNG fuel product. Further, safety may become an issue when larger amounts of LNG fuel product are stored.

There is a need, therefore, for systems and methods for efficiently storing liquefied natural gas. There is also a need for systems and methods for minimizing losses due to vaporization while in storage.

SUMMARY OF THE INVENTION

Embodiments of the present invention provide systems and methods for storing liquefied natural gas. In one embodiment, the natural gas vapor in the storage tank containing the liquefied natural gas may be directed a heat exchange unit attached to the storage tank, cooled by a refrigerant in the heat exchange unit, and then returned to the storage tank. These systems and methods may be used with either land based storage tanks or shipboard applications to minimize boil off losses, to conserve liquefied natural gas vapor generation, or to maintain the storage tanks at a sufficiently cold temperature. In addition to liquefied natural gas, embodiments of the systems and methods may be used with storage tanks containing other types of gases to minimize vapor losses.

In one embodiment, a method of storing liquefied natural gas in a storage tank includes introducing vaporized natural gas in the storage tank into a heat exchange unit; introducing a refrigerant into the heat exchange unit; liquefying the natural gas by exchanging heat with the refrigerant; vaporizing the refrigerant; and returning the liquefied natural gas to the storage tank. In another embodiment, the refrigerant is liquid nitrogen.

In another embodiment, a system for storing liquefied natural gas includes a storage tank containing the liquefied natural gas; heat exchange unit having a container, a heat exchanger, and an opening in fluid communication with the storage tank, wherein the heat exchange unit is configured to condense a vaporized natural gas from the storage tank and to return the condensed natural gas to the storage tank; and a refrigerant source in fluid communication with the heat exchanger. In yet another embodiment, the opening allows inflow of the vaporized natural gas and outflow of the liquefied natural has. In still yet another embodiment, the storage tank includes a port for outflow of the vaporized natural gas and inflow of liquefied natural gas. In still yet another embodiment, the system includes a gas compressor connected between the storage tank and the heat exchange unit.

In yet another embodiment, the heat exchange unit is at least partially disposed in the storage tank. The opening may be positioned inside the storage tank.

In yet another embodiment, the liquid natural gas may be produced from heat exchange with liquid nitrogen or air. The produced liquid natural gas may be used as vehicle fuel. The liquid natural gas produced may be stored in a storage tank equipped with a heat exchange system to minimize losses of the natural gas due to vaporization. In yet anther embodiment, the vaporized liquid nitrogen or air may be routed in the system to regenerate a heat exchange unit and/or a natural gas pretreatment unit. After assisting with the regeneration, the liquid nitrogen or air may be safely vented to atmosphere.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.

FIG. 1 is a process flow diagram of an exemplary embodiment of a gas liquefaction system for producing liquid natural gas.

FIG. 2 is a process flow diagram of another exemplary embodiment of a gas liquefaction system for producing liquid natural gas.

FIG. 3 illustrates an embodiment of a system for storing liquefied natural gas.

FIG. 4 illustrates another embodiment of a system for storing liquefied natural gas.

FIG. 5 illustrates another embodiment of a system for storing liquefied natural gas.

DETAILED DESCRIPTION

FIG. 1 illustrates an exemplary embodiment of a gas liquefaction system 10. The system includes a gas source 100 for supplying a feed gas such as natural gas for liquefaction. The gas source 100 may be connected to a pair of gas pretreatment units 500, 600 for pretreating the feed gas. The pretreatment units 500, 600 may be used to remove any undesired contaminants in the feed gas prior to liquefaction. As shown, the pair of pretreatment units 500, 600 are connected in parallel. In this respect, the pretreatment units 500, 600 may be operated in alternating cycles such that one unit 500 may be in treatment mode, while the other unit 600 is in the regeneration mode.

As shown, the feed gas may be introduced into the system 10 via line 20. Valves 21, 22 may be used to control feed gas flow into the pretreatment units 500, 600. Line 20 may be equipped with a flow control 33 to control the flow of the feed gas in line 20. In one embodiment, natural gas may be introduced into line 20 at a pressure from about 20 psig to about 1200 psig; preferably, about 100 psig to about 350 psig, and at a temperature from about 0° F. to about 120° F.; preferably, from about 80° F. to about 100° F. The natural gas feed may include a hydrocarbon mixture of gases having at least one carbon, such as methane, ethane, propane, butane, pentane, and heavier hydrocarbons. The natural gas feed may also include contaminants such as carbon dioxide, hydrogen sulfide, and water. In one embodiment, the natural gas feed includes at least 40 mole % of methane; preferably, at least 50 mole % of methane; and more preferably, at least 90 mole % of methane. It must be noted that the gas liquefaction system may be used to liquefy other gases such as ethane gas whereby liquid rich ethane is produced. In one embodiment, the ethane gas includes at least 40 mole % of ethane; preferably, at least 50 mole % of ethane; and more preferably, at least 60 mole % of ethane.

In one embodiment, each pretreatment unit 500, 600 may be configured to remove at least one contaminant from the natural gas. The pretreatment units 500, 600 may employ sorbent beds such as regenerable molecular sieves, activated alumina, other suitable adsorbents, and combinations thereof to remove the contaminants. The molecular sieves are effective to remove the contaminants from the natural gas to extremely low levels and to render the natural gas suitable for liquefaction. Suitable molecular sieves may include known molecular sieves that are suitable for dehydration and/or carbon dioxide and adsorb those molecules having a molecular diameter of less than three to five angstroms. The molecular sieves may be regenerated by passing a heated gas through the pretreatment unit to remove the water and carbon dioxide. In another embodiment, the pretreatment units 500, 600 may include an amine unit to assist with contaminant removal. The amine unit may use an aqueous amine-containing solution such as digycolanolamine (DEA) or methyldiethanolamine (MDEA), as well as other types of known physical or chemical solvents to absorb water from the natural gas. In another embodiment, a glycol dehydration unit may be used to remove the contaminants instead of or in addition to the molecular sieve unit. In yet another embodiment, the glycol dehydration unit may be connected downstream from the amine unit.

In an exemplary operation of an alternating cycle, natural gas is introduced into the first pretreatment unit 500 via line 20 through valve 21. Valve 22 is closed to block entry into the second pretreatment unit 600. The first pretreatment unit 500 may be operated to treat the natural gas until the adsorbent beds are spent. When this occurs, the first pretreatment unit 500 is switched to regeneration mode to regenerate the adsorbent beds. In particular, valve 21 is closed and valve 22 is opened. As a result, the natural gas is directed to the second pretreatment unit 600. The second pretreatment 600, which had been in the regeneration mode, is switch to treatment mode to treat the incoming natural gas prior to liquefaction. Operation of the pretreatment units 500, 600 in this alternating cycle allows the system to continuously produce liquefied natural gas. Although an alternating cycle is discussed, it is contemplated that the pretreatment units 500, 600 may be simultaneously active.

After pretreatment, the natural gas is directed to a heat exchange unit for liquefaction. As shown, the system includes two heat exchange units 700, 800 which may be operated simultaneously or in alternating cycles. The pretreated natural gas leaving each pretreatment unit 500, 600 may be directed to either or both heat exchange units 700, 800 via valves 23, 24, 25, 26. The heat exchange units 700, 800 may be selected from any suitable heat exchange unit for liquefying natural gas as is known to a person of ordinary skill. Exemplary heat exchange units include brazed aluminum heat exchangers, plate fin and tube exchangers, plate frame exchangers, welded plate exchangers, compact exchangers, brazed plate heat exchangers, shell and tube exchangers, and other suitable heat exchange units known to a person of ordinary skill.

In one embodiment, the natural gas leaving the first pretreatment unit 500 is directed to the first heat exchange u nit 700 where it exchanges heat with a refrigerant such as liquid nitrogen or air. The pretreated natural gas may pass through valve 23 and enter the first heat exchange unit 700 via flow path 27. At this time, valve 24 is closed to block flow to the second heat exchange unit 800. The refrigerant enters the first heat exchange unit 700 via flow path 28. The refrigerant cools the natural gas sufficiently to cause liquefaction of the natural gas, thereby producing liquid natural gas. In turn, the indirect heat exchange causes vaporization of the liquid nitrogen or air.

In one embodiment, the heat exchange unit 700 may be utilized to remove contaminants from the natural gas through freezing to produce solid products of the contaminants, which may include carbon dioxide, hydrogen sulfide, water, and hydrocarbons having more than five carbons. The solid products may be removed by adherence to a surface of the heat exchanger or filtered out using a filter unit 710 as the liquefied natural gas leaves the heat exchange unit 700. In one embodiment, the filter unit 710 may include a screen to capture the solid contaminant products. Because the heat exchange units 700, 800 may be capable of removing contaminants from the natural gas, it is contemplated that the pretreatment units 500, 600 are optional equipments in the system.

The newly formed liquid natural gas flows from the first heat exchange unit 700 to an insulated storage tank 300 via line 29. Valve 30 may be used to control fluid communication through line 29. Valve 72 is closed to block flow to the second heat exchange unit 800. Line 29 may further be equipped with a temperature/pressure control 31 to help maintain the liquid natural gas heading to the storage tank 300 at a temperature from about 200° F. to about 240° F. Additionally, line 29 may include a flow control 32. Flow control 32 may be linked to flow control 33 for monitoring and adjustment to optimize the liquefaction process. The storage tank 300 may be constructed as a stationary tank or a mobile tank. The pressure of the storage tank 300 may be controlled so that pumping to the liquid natural gas storage tank is not required. In one embodiment, the pressure in the storage tank 300 is from about 40 psig to about 100 psig; preferably, from about 65 psig to about 80 psig. A dispensing unit 400 may be connected to the storage tank 300 to facilitate the fueling of a vehicle or transfer of the liquefied natural gas to mobile storage unit. Alternatively, a pump may be provided to assist with the fueling of the vehicle or transfer of the liquid natural gas.

Refrigerant for cooling the feed gas is supplied from a refrigerant source unit 200 in the system 10. The refrigerant may be selected from liquid nitrogen or liquid air or other suitable material for liquefying the feed gas. The refrigerant may be stored in the refrigerant source unit 200 at a pressure from about 20 psig to about 150 psig and at a temperature from about −300° F. to about −270° F. In one embodiment, the refrigerant is obtained from a commercial vendor at approximately 100 psig. In another embodiment, a refrigerant liquefying unit may be connected to the system 10 to supply the refrigerant. The refrigerant, in this example liquid nitrogen, leaving the source unit 200 initially flows through a valve loop 120 having multiple valves 41, 42, 43, 44, for directing the liquid nitrogen to the appropriate heat exchange unit. In one embodiment, valve 41 is open and valves 42, 44 are closed to direct the liquid nitrogen to the first heat exchange unit 700. The liquid nitrogen may flow through the jacket of the filter unit 710 prior to entering the heat exchange unit 700 via flow path 28. The liquid nitrogen is vaporized to gas after indirectly exchanging heat with the natural gas. The nitrogen gas may leave the heat exchange unit 700 at a temperature from about 70° F. to 110° F.

The vaporized nitrogen may be used to regenerate (also referred to as derime) the second heat exchange unit 800. As discussed above, the second heat exchange unit 800 may be in regeneration mode while the first heat exchange unit 700 is in operation. The second heat exchange unit 800 may have collected sufficient frozen contaminants during operation to adversely affect its effectiveness. The warm nitrogen from the first heat exchange unit 700 is directed via line 45 to flow path 48 of the second heat exchange unit 800 to cause sublimation of the frozen contaminants. In this respect, solid contaminants accumulated on the heat exchange surfaces in flow path 47 may be removed, thereby restoring the effectiveness of the second heat exchange unit 800. The warm nitrogen may also flow through the jacket of the filter unit 810 connected to the second heat exchange unit 800. The warm nitrogen may similarly derime the filter unit 810. Thereafter, the warm nitrogen flows back to the valve loop 120, where it is directed to the second pretreatment unit 600 to facilitate regeneration thereof. As shown, the nitrogen gas flows through valve 43 of the valve loop 120 and is directed via line 51 to the second pretreatment unit 600. Valve 25 is open for communication to the unit 600 and valve 26 is closed to block communication to unit 800.

In one embodiment, a pulse regeneration process may be used to regenerate the second pretreatment unit 600. During operation, the adsorbents in the second pretreatment unit 600 may have retained a mixture of carbon dioxide, water, natural gas, and other contaminants. This mixture adversely affects the operation of the pretreatment unit 600 and is preferably removed to regenerate the unit 600. The regeneration process may include initially placing the second pretreatment unit 600 in fluid communication with a fuel storage unit 150. This step requires opening valve 61 and closing valves 62, 65, and 67. Gases such as methane are allowed to flow to the fuel storage unit 150 for a short period of time, for example, from about 5 seconds to about 5 minutes; preferably from about 10 seconds to 45 seconds. Directing the flow of these gases to the fuel storage unit 150 may eliminate the discharge of hazardous material into the atmosphere while capturing these gases for use as fuel. Thereafter, the line between the second pretreatment unit 600 and the fuel storage unit 150 is closed, and the vent line to vent nitrogen to atmosphere is opened by opening valves 62 and 65 and closing valve 66.

After the vent line is opened, warm nitrogen from the second exchange unit 800 is supplied to flush contaminants such as carbon dioxide and water from the adsorbent beds. In one embodiment, the warm nitrogen may be heated by an optional heater 160 prior to entering the second pretreatment unit 600. The heater 160 may be configured to heat the nitrogen to a temperature from about 80° F. to about 550° F.; preferably, from about 80° F. to about 450° F. Heated nitrogen is supplied to purge the contaminants from the second pretreatment unit 600 for a period of time from about 2 minutes to about 1,000 minutes; preferably, from about 100 minutes to about 180 minutes; and more preferably, from about 60 minutes to about 150 minutes.

After purging with heated nitrogen, the temperature of the nitrogen is decreased to cool the adsorbent beds. The heater 160 may be turned off or reduced to allow the nitrogen to cool. The cooler nitrogen is allowed to flow for a period from about from about 2 minutes to about 1,000 minutes; preferably, from about 45 minutes to about 100 minutes. At the end of the cooling period, the regeneration process is complete and the second pretreatment unit 600 may be returned to operation. In one embodiment, the pretreatment units 500, 600 are operated such that the timing for switching the active pretreatment unit to regeneration mode depends on whether the pretreatment unit already in regeneration mode is ready to become active. In another embodiment, the pretreatment unit in regeneration mode may become active before the first pretreatment unit is switched to regeneration mode so that both pretreatment units are active simultaneously.

Fuel for the heater 160 may be supplied from the fuel storage unit 150. As shown, valves 76 and 77 control communication between the storage unit 150 and the heater 160. Fuel in the storage unit 150 may be replenished by diverting a portion of the natural gas leaving the pretreatment units 500, 600. For example, natural gas may be diverted from the first pretreatment unit 500 via line 71 and directed through valve 73 and up flow path 47 of the second heat exchange unit 800. In this respect, the natural gas may be used to flush out heavier hydrocarbons accumulated in the heat exchange unit 800. From there, the natural gas may flow through valve 74 and toward the fuel storage unit 150. Alternatively, the natural gas may be diverted through valve 75 and flowed to the storage unit 150.

In operation, the gas liquefaction system 10 may be used to produce liquid natural gas. In one example, natural gas may be supplied to the system at a temperature of about 100° F. and a pressure of about 100 psia. After processing in the first pretreatment unit 500, the natural gas is introduced into the first heat exchange unit 700. Liquid nitrogen, acting as the refrigerant, may be supplied at a temperature of about −283° F. and a pressure of about 100 psia to the first heat exchange unit 700 to liquefy the natural gas. The natural gas leaves the first heat exchange unit 700 in liquid form at a temperature of about −208° F. and a pressure of about 95 psia.

FIG. 2 illustrates another embodiment of a process flow diagram of a gas liquefaction system 210. In this embodiment, the heat exchange units 700, 800 are configured to facilitate heat exchange and remove contaminants from the natural gas. In this respect, this system does not require a pretreatment unit, but may nevertheless, include one. For clarity purposes, components in FIG. 2 that are similar to FIG. 1 have been labeled with the same reference number and may not be described in detail.

The system includes a gas source 100 for supplying a feed gas such as natural gas for liquefaction. The gas source 100 is connected to heat exchange units 700, 800. As shown, the pair of heat exchange units 700, 800 are connected in parallel. The heat exchange units 700, 800 may be operated in alternating cycles such that one unit 700 may be in liquefaction mode, while the other unit 800 is in the regeneration mode. In another embodiment, the heat exchange units 700, 800 may be operated on the same cycle such at both units 700, 800 are in the liquefaction mode. The heat exchange units 700, 800 may be selected from any suitable heat exchange unit for liquefying natural gas as is known to a person of ordinary skill. Exemplary heat exchange units include brazed aluminum heat exchangers, plate fin and tube exchangers, plate frame exchangers, welded plate exchangers, compact exchangers, brazed plate heat exchangers, shell and tube exchangers, and other suitable heat exchange units known to a person of ordinary skill.

As shown, the feed gas may be introduced into the system 210 via line 20. Valves 221, 222 may be used to control feed gas flow into the heat exchange units 700, 800. The natural gas may be directed to either or both heat exchange units 700, 800 via valves 221, 222. Line 20 may be equipped with a flow control 33 to control the flow of the feed gas in line 20. In one embodiment, natural gas may be introduced into line 20 at a pressure from about 20 psig to about 1200 psig; preferably, about 100 psig to about 350 psig, and at a temperature from about 0° F. to about 120° F.; preferably, from about 80° F. to about 100° F. The natural gas feed may include a hydrocarbon mixture of gases having at least one carbon, such as methane, ethane, propane, butane, pentane, and heavier hydrocarbons. The natural gas feed may also include contaminants such as carbon dioxide, hydrogen sulfide, and water.

In one embodiment, the natural gas entering first heat exchange unit 700 exchanges heat with a refrigerant such as liquid nitrogen or air. The natural gas may enter the first heat exchange unit 700 via flow path 27. At this time, valve 222 is closed to block flow to the second heat exchange unit 800. The refrigerant enters the first heat exchange unit 700 via flow path 28. The refrigerant cools the natural gas sufficiently to cause liquefaction of the natural gas, thereby producing liquid natural gas. In turn, the refrigerant absorbs heat from the natural gas, which causes vaporization of the liquid nitrogen or air.

In one embodiment, the heat exchange unit 700 may be utilized to remove contaminants from the natural gas by freezing the contaminants to produce solid products of the contaminants, which may include carbon dioxide, hydrogen sulfide, water, and hydrocarbons having more than five carbons. The solid products may be removed by adherence to a surface of the heat exchanger or filtered out using a filter unit 710 as the liquefied natural gas leaves the heat exchange unit 700. In one embodiment, the filter unit 710 may include a screen to capture the solid contaminant products. Even though the heat exchange units 700, 800 may be capable of purifying the natural gas, the system 210 may optionally include pretreatment unit to assist with removing contaminants in the natural gas.

The newly formed liquid natural gas in the first heat exchange unit 700 is directed to an insulated storage tank 300 via line 29, as discussed above with respect to FIG. 1. Valve 30 may be used to control fluid communication through line 29. Valve 72 is closed to block flow to the second heat exchange unit 800. Line 29 may further be equipped with a temperature/pressure control 31 to help maintain the liquid natural gas heading to the storage tank 300 at a temperature from about 200° F. to about 240° F. Additionally, line 29 may include a flow control 32. Flow control 32 may be linked to flow control 33 for monitoring and adjustment to optimize the liquefaction process. The storage tank 300 may be constructed as a stationary tank or a mobile tank. The pressure of the storage tank 300 may be controlled so that pumping to the liquid natural gas storage tank is not required. In one embodiment, the pressure in the storage tank 300 is from about 40 psig to about 100 psig; preferably, from about 65 psig to about 80 psig. A dispensing unit 400 may be connected to the storage tank 300 to facilitate the fueling of a vehicle or transfer of the liquefied natural gas to mobile storage unit. Alternatively, a pump may be provided to assist with the fueling of the vehicle or transfer of the liquid natural gas.

Refrigerant for cooling the feed gas is supplied from a refrigerant source unit 200 in the system 210. The refrigerant may be selected from liquid nitrogen or liquid air or other suitable material for liquefying the feed gas. The refrigerant, in this example, liquid nitrogen, leaving the source unit 200 initially flows through a valve loop 120 having multiple valves 41, 42, 43, 44, for directing the liquid nitrogen to the appropriate heat exchange unit. In one embodiment, valve 41 is open and valves 42, 44 are closed to direct the liquid nitrogen to the first heat exchange unit 700. The liquid nitrogen may flow through the jacket of the filter unit 710 prior to entering the heat exchange unit 700 via flow path 28. The liquid nitrogen is vaporized to gas after absorbing heat from the natural gas. The nitrogen gas may leave the heat exchange unit 700 at a temperature from about 70° F. to 110° F.

The vaporized nitrogen may be used to regenerate (also referred to as derime) the second heat exchange unit 800. As discussed above, the second heat exchange unit 800 may be in regeneration mode while the first heat exchange unit 700 is in operation. The second heat exchange unit 800 may have collected sufficient frozen contaminants during operation to adversely affect its effectiveness. The warm nitrogen from the first heat exchange unit 700 is directed via line 45 to flow path 48 of the second heat exchange unit 800 to cause sublimation of the frozen contaminants. In this respect, solid contaminants accumulated on the heat exchange surfaces in flow path 47 may be removed, thereby restoring the effectiveness of the second heat exchange unit 800. The warm nitrogen may also flow through the jacket of the filter unit 810 connected to the second heat exchange unit 800. The warm nitrogen may similarly derime the filter unit 810. Thereafter, the warm nitrogen flows back to the valve loop 120, where it is directed to the vent line 51. As shown, the nitrogen gas flows through valve 43 of the valve loop 120 and is directed to line 51 for venting.

In one embodiment, natural gas may be diverted from the feed line 20 to assist with purging of natural gas flow path of exchange unit in regeneration mode. For example, natural gas from line 20 may be diverted to line 71 and directed through valve 73 and up flow path 47 of the second heat exchange unit 800. In this respect, the natural gas may be used to flush out heavier hydrocarbons and/or contaminants such as carbon dioxide and water accumulated in the heat exchange unit 800. After purging the heat exchange unit 800, the natural gas may flow through valve 74 and toward the fuel storage unit 150.

At the end of the regeneration process, the second heat exchange unit 800 may be returned to operation. In one embodiment, the heat exchanged units 700, 800 are operated such that the timing for switching the active heat exchange unit 700 to regeneration mode depends on whether the heat exchange unit 800 already in regeneration mode is ready to become active. In another embodiment, the heat exchange unit in regeneration mode may become active before the first heat exchange unit is switched to regeneration mode so that both heat exchange units are active simultaneously.

Natural gas in the fuel storage unit 150 may be used as fuel by a power generator 900 to generate energy for consumption. Fuel in the storage unit 150 may also be replenished by diverting a portion of the natural gas in line 20 via valve 265. Alternatively, the natural gas may be diverted through valve 75 and flowed to the storage unit 150.

Embodiments of the present invention also provide systems and methods for storing liquefied natural gas. In one embodiment, the natural gas vapor in the storage tank containing the liquefied natural gas may be directed a heat exchange unit attached to the storage tank, cooled by a refrigerant in the heat exchange unit, and then returned to the storage tank. These systems and methods may be used with either land based storage tanks or shipboard applications to minimize boil off losses, to conserve liquefied natural gas vapor generation, or to maintain the storage tanks at a sufficiently cold temperature. In addition to liquefied natural gas, embodiments of the systems and methods may be used with storage tanks containing other types of gases to minimize vapor losses.

FIG. 3 shows an exemplary embodiment of a heat exchange unit 750 connected to a storage tank 730. The storage tank 730 may be used to contain a gas in liquefied state, such as liquefied natural gas. The storage tank 730 may be disposed on land or on a floating vessel. In one embodiment, the storage tank 730 is the LNG storage tank 300 shown in FIG. 1. The storage tank 730 may include an inlet 731 for introducing the liquefied natural gas and an outlet 732 for dispensing the liquefied natural gas. The storage tank 730 may also include at least one port 735 for connection to and fluid communication with the heat exchange unit.

The heat exchange unit 750 is configured to cool and condense vaporized liquefied natural gas in the storage tank 730. In one embodiment, the heat exchange unit 750 includes a heat exchanger 760 disposed in a container 770 for receiving the vaporized liquefied natural gas. The vaporized liquefied natural gas may exchange heat with the fluid flowing in the heat exchanger 760, thereby cooling the vaporized liquefied natural gas. In a preferred embodiment, sufficient energy is transferred to liquefy the vaporized natural gas.

In one embodiment, the container 770 includes a port 775 connected to the port 735 of the storage tank 730. The connected ports 735, 775 form a fluid path that allows movement of the liquefied natural gas in either direction. For example, the vaporized natural gas may flow into the container 770 through the fluid path, while condensed liquefied natural gas in the container 770 may return to the storage tank 730 through the same path. In another embodiment, the container 770 and storage tank 730 may have separate fluid paths for the vaporized liquefied natural gas and the condensed liquefied natural gas. In yet another embodiment, the container 770 and the storage tank 730 may have multiple fluid paths for any combination of integrated or segregated movement of the vaporized and liquefied natural gas. The container 770 and the storage tank 730 may be arranged to facilitate fluid flow through the ports 735, 775. As shown, the port 775 of the heat exchange unit 750 is positioned above the port 735 of the storage tank 730. Additionally, the port 735 of the storage tank 730 is positioned at an upper portion of the storage tank 730, and the port 775 of the container 770 is positioned at a lower portion of the container 770. In this respect, vaporized liquefied natural gas is allowed to freely flow upward into the heat exchange unit 750, while the condensed liquefied natural gas returns to the storage tank 730 under the assistance of gravity. In yet another embodiment, the pressure in the container 770 is maintained at a lower pressure than storage tank 730. In another embodiment, the heat exchange unit 750 is configured as an add-on to existing storage tanks. The container 770 of the heat exchange unit 750 may have a smaller volume size than the storage tank 730.

The heat exchanger 760 is adapted to circulate a heat transfer fluid into the container 770 for transferring heat with the liquefied natural gas. The heat exchanger 760 includes an inlet 761 for receiving the heat transfer fluid from an exterior source and an outlet 762 for dispensing the heat transfer fluid out of the container 770. The heat exchanger 760 may be selected from any suitable heat exchangers for liquefying natural gas as is known to a person of ordinary skill. Exemplary heat exchanger 760 include brazed aluminum heat exchangers, plate fin and tube exchangers, plate frame exchangers, welded plate exchangers, compact exchangers, brazed plate heat exchangers, shell and tube exchangers, and other suitable heat exchange units known to a person of ordinary skill.

In one embodiment, liquid nitrogen may be used as the heat transfer fluid to condense the vaporized liquefied natural gas in the container 770. The heat transfer process may be configured such that enough energy is provided by the liquid nitrogen to liquefy the liquefied natural gas. The heat transfer may cause the liquid nitrogen to vaporize. The vaporized nitrogen directed through the outlet 762 may be removed by venting to atmosphere. The liquid nitrogen may be supplied from a tank, generated from an onsite liquefier, or provided from any other suitable source. Referring back to FIG. 1, liquid nitrogen stored in refrigerant source unit 200 may be used as the source for a heat exchange unit 750 attached to the storage tank 300.

In yet another embodiment, uncondensed vaporized natural gas may optionally flow out of container 770 through an outlet 766 connected to a gas compressor. The gas leaving the gas compressor may be directed to another heat exchanger for liquefaction before returning to the storage tank 730.

In operation, the storage tank 730 contains liquefied natural gas at a temperature from about 200° F. to about 240° F. The storage tank 730 also contains vaporized natural gas due to vaporization of the liquefied natural gas. The vaporized natural gas is allowed to flow into the container 770 through the ports 735, 775. Liquid nitrogen is supplied from a refrigerant source into the heat exchanger 760. The liquid nitrogen may be supplied at a pressure from about 20 psig to about 150 psig and a temperature from about −300° F. to about −270° F. The liquid nitrogen indirectly exchanges heat with the natural gas to cool and condense the vaporized natural gas in the container 770. The condensed natural gas may be at a temperature from about 200° F. to about 240° F. The condensed natural gas flows back to the storage tank 730 in liquid form. The liquid nitrogen is vaporized after indirectly exchanging heat with the natural gas. The vaporized nitrogen directed through the outlet 762 and vented to atmosphere. In this manner, boil off of losses of the liquefied natural gas in the storage tank 730 may be minimized. The condensed natural gas returning from the heat exchange unit 750 may assist with maintaining the storage tank 730 at a sufficient cold temperature to minimize boil off of the liquefied natural gas.

FIG. 4 shows another embodiment of a heat exchange unit 850 connected to a storage tank 730. Similar features shown in FIG. 4 that are similar to features shown in FIG. 3 are designated with the same reference numbers and will not be described in detail. The storage tank 730 may be used to contain a gas in liquefied state, such as liquefied natural gas. The storage tank 730 may include an inlet 731 for introducing the liquefied natural gas and an outlet 732 for dispensing the liquefied natural gas. The storage tank 730 may also include at least one port 735 for connection to and fluid communication with the heat exchange unit 850.

The heat exchange unit 850 is configured to cool and condense vaporized liquefied natural gas in the storage tank 730. In one embodiment, the heat exchange unit 850 includes a heat exchanger 760 disposed in a container 870 for receiving the vaporized liquefied natural gas. The heat exchanger 760 supplies the heat transfer fluid for condensing the vaporized natural gas. In a preferred embodiment, sufficient energy is transferred to liquefy the natural gas.

In one embodiment, the container 870 includes an upper port 875 connected to the port 735 of the storage tank 730. The fluid path 836 to the upper port 875 may be used to direct vaporized natural gas to the heat exchange unit 850. The container 870 also includes a lower port 876 connected to the port 735 of the storage tank 730. The lower port 876 may be used to direct condensed natural gas back to the storage tank 730. In this respect, entry of the vaporized natural gas into the heat exchange unit 850 is separated from the condensed natural gas. The lower port 876 is positioned above the port 735 to facilitate return to the storage tank 730. In yet another embodiment, the fluid paths to either ports 875, 876 may be configured to allow the ingress or egress of either or both the vaporized natural gas and the condensed natural gas. In yet another embodiment, the pressure in the container 870 is maintained at a lower pressure than storage tank 730. In another embodiment, the heat exchange unit 850 is configured as an add-on to existing storage tanks. The container 870 of the heat exchange unit 750 may have a smaller volume size than the storage tank 730. For example, the container 870 may be sufficiently sized for use as a portable add-on to the storage tank 730. In yet another embodiment, the heat exchange unit 850 may be connected to the storage tank 730 without the use of compressors or pumps to assist with the flow of the natural gas to and from the storage tank 730.

In yet another embodiment, an optional gas compressor 890 may be utilized to facilitate movement of the vaporized natural gas. For example, the gas compressor 890 may be positioned in the entry path 836 to the heat exchange unit 850. Exemplary gas compressors include any suitable gas compressor known to a person of ordinary skill in the art.

In one embodiment, liquid nitrogen may be used as the heat transfer fluid to condense the vaporized liquefied natural gas in the container 870. The heat transfer process may be configured such that enough energy is provided by the liquid nitrogen to liquefy the liquefied natural gas. The heat transfer may cause the liquid nitrogen to vaporize. The vaporized nitrogen directed through the outlet may be removed by venting to atmosphere. The liquid nitrogen may be supplied from a tank, generated from an onsite liquefier, or provided from any other suitable source. Referring back to FIG. 1, liquid nitrogen stored in refrigerant source unit 200 may be used as the source for a heat exchange unit attached to the storage tank 300.

In operation, the storage tank 730 contains liquefied natural gas at a temperature from about 200° F. to about 240° F. The storage tank 730 also contains vaporized natural gas due to vaporization of the liquefied natural gas. The vaporized natural gas is allowed to flow into the container 870 through the port 875. Liquid nitrogen is supplied from a refrigerant source into the heat exchanger 760. The liquid nitrogen indirectly exchange heat with the natural gas to cool and condense the vaporized natural gas in the container 870. The condensed natural gas may be at a temperature from about 200° F. to about 240° F. The condensed natural gas leaves the container 870 through port 876 and flows back to the storage tank 730 in liquid form. The liquid nitrogen is vaporized after indirectly exchanging heat with the natural gas. The vaporized nitrogen directed through the outlet 762 and vented to atmosphere. In this manner, boil off of losses of the liquefied natural gas in the storage tank 730 may be minimized.

FIG. 5 illustrates another embodiment of a heat exchange unit 950 connected to a storage tank 930. The storage tank 930, partially shown, may be used to contain a gas in liquefied state, such as liquefied natural gas. The storage tank 930 may be disposed on land or on a floating vessel. In one embodiment, the storage tank 930 is the LNG storage tank 300 shown in FIG. 1. The storage tank 930 may also include at least one port 935 for connection to the heat exchange unit 950.

The heat exchange unit 950 is configured to cool and condense vaporized liquefied natural gas in the storage tank 930. In one embodiment, the heat exchange unit 950 includes a heat exchanger 960 disposed in a container 970 for receiving the vaporized liquefied natural gas. The vaporized natural gas may exchange heat with the refrigerant fluid flowing in the heat exchanger 960, thereby cooling the vaporized liquefied natural gas. In a preferred embodiment, sufficient energy is transferred to liquefy the vaporized liquefied natural gas.

In one embodiment, the heat exchange unit 950 is a compact unit that is at least partially positionable in the storage tank 930. In the embodiment shown in FIG. 5, the container 970 is at least partially disposed in the storage tank 930 through the port 935 of the storage tank 930. The container 970 may be any shape suitable for positioning through the port 935, for example, cylindrical shape. The container 970 may include an inlet opening 975 for fluid communication between the storage tank 930 and the interior of the container 970. In one embodiment, the inlet opening 975 is formed below the top surface of the port 935. The container 970 may also include outlet opening 976 at a lower portion of the container 970. In one embodiment, the outlet opening 976 is extended by a tubular 978. Vaporized liquefied natural gas may flow into the container 970 through the inlet opening 975, while condensed liquefied natural gas in the container 970 may flow out of the container 970 through the outlet opening 976. It is contemplated the natural gas, either gas or liquid form, may flow into or out of the inlet or outlet openings 975, 976 or both.

The heat exchanger 760 is adapted to circulate a heat transfer fluid into the container 970 for transferring heat with the liquefied natural gas. The heat exchanger 960 includes an inlet 961 for receiving the heat transfer fluid from an exterior source and an outlet 962 for dispensing the heat transfer fluid out of the heat exchanger 960. The heat exchanger 960 is configured for at least partial positioning in the container 970. In one embodiment, the heat exchanger 960 is positioned below the inlet opening 975 and above the outlet opening 976. The heat exchanger may be selected from any suitable heat exchangers for liquefying natural gas as is known to a person of ordinary skill. Exemplary heat exchanger 960 include brazed aluminum heat exchangers, plate fin and tube exchangers, plate frame exchangers, welded plate exchangers, compact exchangers, brazed plate heat exchangers, shell and tube exchangers, and other suitable heat exchange units known to a person of ordinary skill.

In one embodiment, liquid nitrogen may be used as the heat transfer fluid to condense the vaporized liquefied natural gas in the container 970. The heat transfer process may be configured such that enough energy is provided by the liquid nitrogen to liquefy the liquefied natural gas. The heat transfer may cause the liquid nitrogen to vaporize. The vaporized nitrogen directed through the outlet 962 may be removed by venting to atmosphere. The liquid nitrogen may be supplied from a tank, generated from an onsite liquefier, or provided from any other suitable source. Referring back to FIG. 1, liquid nitrogen stored in refrigerant source unit 200 may be used as the source for a heat exchange unit 950 attached to the storage tank 300.

In operation, the storage tank 930 contains liquefied natural gas at a temperature from about 200° F. to about 240° F. The storage tank 930 also contains vaporized natural gas due to vaporization of the liquefied natural gas. The vaporized natural gas is allowed to flow into the container 970 through the inlet opening 975. Liquid nitrogen is supplied from a refrigerant source into the heat exchanger 960. The liquid nitrogen may be supplied at a pressure from about 20 psig to about 150 psig and a temperature from about −300° F. to about −270° F. The liquid nitrogen indirectly exchanges heat with the natural gas to cool and condense the vaporized natural gas in the container 970. The condensed natural gas may be at a temperature from about 200° F. to about 240° F. The condensed natural gas drains out of the container 970 through the outlet opening 976 in liquid form. The liquid nitrogen is vaporized after indirectly exchanging heat with the natural gas. The vaporized nitrogen directed through the outlet 962 and vented to atmosphere. In this manner, boil off of losses of the liquefied natural gas in the storage tank 930 may be minimized. The condensed natural gas returning from the heat exchange unit 950 may assist with maintaining the storage tank 930 at a sufficient cold temperature to minimize boil off of the liquefied natural gas.

While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow. 

1. A method of storing liquefied natural gas in a storage tank, comprising: introducing vaporized natural gas in the storage tank into a heat exchange unit; introducing a refrigerant into the heat exchange unit; liquefying the natural gas by exchanging heat with the refrigerant; vaporizing the refrigerant; and returning the liquefied natural gas to the storage tank.
 2. The method of claim 1, further comprising providing a first fluid path for flowing the vaporized natural gas from the storage tank and introducing the vaporized natural gas into the heat exchange unit.
 3. The method of claim 2, wherein the liquefied natural gas is returned to the storage tank through a second fluid path in fluid communication with the first fluid path.
 4. The method of claim 2, wherein the liquefied natural gas is returned to the storage tank through a second fluid path not in fluid communication with the first fluid path.
 5. The method of claim 2, further comprising providing a gas compressor in the first fluid path.
 6. The method of claim 1, wherein the vaporized natural gas is introduced into the heat exchange unit through the same port as the liquefied natural gas leaving the heat exchange unit.
 7. The method of claim 1, wherein the returning liquefied natural gas reduces a temperature in the storage tank.
 8. The method of claim 1, wherein heat exchange unit is at least partially disposed in the storage tank.
 9. The method of claim 8, wherein the vaporized natural gas is introduced into the heat exchange unit through an inlet opening of the heat exchange unit that is located inside the storage tank.
 10. The method of the claim 9, wherein the liquefied natural gas leaves the heat exchange unit through an outlet opening that is located inside the storage tank.
 11. The method of claim 1, wherein the refrigerant comprises liquid nitrogen.
 12. The method of claim 11, further comprising venting the vaporized nitrogen to atmosphere.
 13. The method of claim 1, further comprising venting the vaporized refrigerant to atmosphere.
 14. A system for storing liquefied natural gas, comprising: a storage tank containing the liquefied natural gas; heat exchange unit having: a container; a heat exchanger; and an opening in fluid communication with the storage tank, wherein the heat exchange unit is configured to condense a vaporized natural gas from the storage tank and to return the condensed natural gas to the storage tank; and a refrigerant source in fluid communication with the heat exchanger.
 15. The system of claim 14, wherein the opening allows inflow of the vaporized natural gas and outflow of the liquefied natural has.
 16. The system of claim 14, wherein the storage tank includes a port for outflow of the vaporized natural gas and inflow of liquefied natural gas.
 17. The system of claim 14, further comprising a gas compressor connected between the storage tank and the heat exchange unit.
 18. The system of claim 14, wherein the heat exchange unit is at least partially disposed in the storage tank.
 19. The system of claim 18, wherein the opening is positioned inside the storage tank.
 20. The system of claim 14, wherein the refrigerant is liquid nitrogen. 